In mobile telecommunication systems there always exists frequency offset in a signal transmitted from a radio transmitter to a radio receiver. The frequency offset may be caused by a frequency difference between oscillators of the transmitter and the receiver, but a major cause for the frequency offset is also the Doppler shift affecting the signal in a mobile environment. The Doppler shift is caused by a change in the distance between a mobile terminal and a base station due to the movement of the mobile terminal.
Frequency offsets typically leak to a baseband part of the receiver, thereby causing a phase rotation, i.e. frequency error, in a received baseband signal. This frequency error has to be compensated for in order to ensure a reliable detection of received data. First of all, the frequency error introduced into the received signal is estimated and, thereafter, the phase rotation in the received baseband signal is compensated for by weighting the received signal with estimated phase rotation values which are used for rotating the phase of the received signal to the opposite direction.
A conventional solution for estimating the frequency error is to estimate a phase difference between pilot symbols transmitted at different time instants. Then, a frequency error estimate is obtained by dividing the phase difference between the pilot symbols by a time difference between transmission times of the respective pilot symbols. The estimation is based on the fact that the phase difference within a determined time period represents a frequency offset. In theory, the frequency is a time derivative of a phase.
A conventional solution does not, however, provide a sufficient performance in all applications. As an example, let us consider a current frame structure of a downlink of a UMTS LTE (Universal Mobile Telecommunication System Long-Term Evolution) standardized within the 3GPP (3rd Generation Partnership Project). The downlink of the UMTS LTE system is based on OFDMA (Orthogonal Frequency Division Multiple Access) in which a pilot symbol is transmitted periodically on a given subcarrier. According to the current standard, the pilot symbol is transmitted on the same subcarrier with seven OFDM symbol intervals meaning that every seventh OFDM symbol has a pilot symbol on the same subcarrier. In OFDM systems, the frequency error should be estimated from pilot symbols transmitted on the same subcarrier, since different frequencies, i.e. different subcarriers, have different phasing properties.
According to the conventional frequency error estimation method, the phase difference between a first pilot symbol transmitted on a subcarrier of a first OFDM symbol and a second pilot symbol transmitted on the same subcarrier of another OFDM symbol transmitted 7 OFDM symbols after the first OFDM symbol is estimated. A time difference between the transmissions of the OFDM symbols is 0.5 ms according to the current standard and, therefore, the maximum frequency error which can be detected with the conventional method is ±1 kHz. This range may not be sufficient and calls for improvement. After all, accurate estimation of even large frequency errors is a key factor in providing reliable data transmission in OFDM systems in which subcarriers are allocated very close to each other in a frequency domain.